The present invention relates to a disc reproducing apparatus for reproducing information recorded on a CD (Compact Disc), a DVD (Digital Video Disc or Digital Versatile Disc).
Usually, in order to ensure a conformability between a recording medium such as CD or DVD (hereinafter simply referred to as disc) and a disc reproducing apparatus, data is recorded on a disc in accordance with a predetermined data format, while recorded data is reproduced by performing an encoding treatment in accordance with the above data format.
For example, as shown in FIG. 5, with regard to a CD, information data is recorded along one spiral-like information track arranged in a program area between a lead-in position and a lead-out position. Further, as shown in FIG. 6A, other data than musical information is recorded in 98 frames forming one block unit. In fact, each frame includes synchronizing signal, indication signal, parity signal and information data.
Further, as shown in FIG. 6A, the indication signal includes P, Q, R, S, T, U, V and W signals each consisting of one bit, and these signals are divided into 8 sub-coding channels. In this way, as shown in FIG. 6B, by virtue of the Q signals included in 98 frames, it is allowed to obtain sub-code Q signals indicating address information (time information) recorded in each block, and synchronizing signals (sub-coding synchronism) S0, S1.
In order to correctly read data recorded on a disc, it is necessary to perform a tracking control of an optical pickup, and to determine whether the address information (time information) is being continuously obtained in accordance with the sub-code Q signals.
When the address information (time information) is being continuously obtained, it is determined that the optical pickup is properly tracing an information track and that data recorded on the disc is being exactly read out, thereby permitting the information reading process to be continued.
On the other hand, if the address information (time information) being continuously obtained has suddenly jumped, it is determined that the optical pickup has deviated to an adjacent information track because of an external vibration or due to some other reasons. At this moment, a block of the track where the optical pickup had arrived immediately before the pickup deviation occurred will be searched, so that the information reading process will be continued from the searched position, thereby effecting a necessary compensation treatment to ensure that the recorded data may be read out completely.
FIG. 7 is a flowchart indicating an example of a compensation treatment according to a prior art. As shown in FIG. 7, when an optical pickup is operated to start its scanning operation, sub-code Q signals will be reproduced from the indication signals of one block (98 frames) at a step S100. Then, at a step S101, a sub-code signal Q reproduced during the last operation is read out from a memory. At a step S102, address information (time information) t1 contained in the last time sub-code signal Q is compared with address information contained in the latest sub-code signal Q so as to obtain a time difference .vertline.t2-t1.vertline.. At a step S103, it is determined whether the time difference .vertline.t2-t2.vertline. has continued for over a predetermined time T HD.
If .vertline.t2-t1.vertline.&lt;T HD, it is determined that the optical pickup is tracing an information track in a correct manner, so that the obtained latest sub-code signal Q is written and stored in the above memory (step S104), thus allowing the normal reproducing operation to be continued at a step S105, thereby repeating the same process beginning from the step S100.
On the other hand, if it is determined at the step S103 that .vertline.t2-t1.vertline..gtoreq.T HD, it may be determined that the optical pickup has jumped (deviated) from one information track to an adjacent track. Then, at a step 106, in accordance with address information t1 of the sub-code signal Q formerly read out from the memory, a block of the track at which the pickup had arrived immediately before the pickup deviation occurred will be searched, so that the information reading process will be continued from the searched position, thereby effecting a compensation treatment to ensure that the recorded data may be read out completely.
However, with the compensation treatment carried out in the above conventional disc reproducing apparatus, there is a problem that although the optical pickup has in fact not jumped (deviated) to an adjacent information track, a mistaken determination will sometimes happen indicating that such a jump (deviation) of the optical pickup has occurred.
Here, a mistaken determination associated with the prior art will be explained in the following with reference to FIG. 5.
Referring to FIG. 5, a virtual line Y has been drawn extending from the center of a CD in the radial direction thereof. As may be understood in FIG. 5, there are fewer blocks toward the inner lead side and there are more blocks toward the outer lead side. For example, the inmost track contains blocks arranging from a first block d1 to a No. m block dm, while an outer track contains blocks arranging from a block D1 to a No. n block Dn, with m being smaller than n, i.e., n&gt;m.
Conventionally, to detect an occurrence of a jump (or deviation) of an optical pickup (hereinafter may be referred to as track jump), the above predetermined time T HD is set to be shorter than a time necessary for scanning (at a predetermined linear speed) blocks d1-dm on the inmost track. In other words, a time necessary for scanning (at a predetermined linear speed) blocks fewer than blocks d1-dm on the inmost track is set to be the predetermined time T HD.
In this way, when an optical pickup jumps from the inmost track to an adjacent track, almost m blocks fail to be scanned. Thus, at the step 103 of FIG. 7, it is allowed to detect whether there is a jump (or deviation) of the optical pickup by comparing a time difference (between a sub-code signal Q of a block immediately before the jump and a sub-code signal Q of a block immediately after the jump) with the above predetermined time T HD.
If a track jump happens when the optical pickup is scanning a track on an outer lead side, the blocks failing to be scanned will be more than those on an inner track. Therefore, it is possible to determine whether there is a track jump at the step S103 shown in FIG. 7.
However, if a lot of defect blocks are continuously existing on a track on the outer lead side, and if the defect blocks are more than the blocks corresponding to the predetermined fixed time T HD, a mistaken determination indicating a track jump will happen regardless of a fact that there is no track jump at all.
For example, when there are blocks D1-Dn existing on the track adjacent to the outer lead side, and blocks X1-Xp among the blocks D1-Dn are defect blocks, it is impossible to correctly read sub-code signal Q from the blocks X1-Xp.
A time difference between a sub-code signal Q of block Xf (which is immediately before a block X1) and another sub-code signal Q of Xe (which is immediately after a block Xp), will become clearly larger than the above predetermined fixed time T HD. As a result, at the step S103 (FIG. 7) there will be a mistaken determination indicating that a track jump has occurred, causing re-start of information reading from a block Xf, regardless of the fact that there is no track jump at all.
Further, even if the information reading is re-started from the block Xf, since the blocks X1-Xp are defect blocks, there will again be a mistaken determination indicating that there has occurred a track jump. In this manner, a scanning treatment for information reading will be repeated again and again from block Xf to block Xe, resulting in a problem that it will become impossible to perform a desired normal reading of information from the block Xe onward.
Further, as to why there will occur defect blocks, a possible reason may be considered to be a wound formed or a dust attached on the optical disc. Alternatively, a reason responsible for a defect block may be that the disc itself as a whole is defect which is often a problem resulting from the disc manufacturing process.
In addition, as to why there will be a mistaken determination, some other reasons may be a deviation in focusing control and a deviation in tracking control of an optical pickup, a jitter caused due to disc skew, or an aging phenomenon of an optical pickup.